Much like guided missiles that sense the heat of a airplane's
engine, sperm are guided to the fertilization zone by temperature,
shows a Weizmann Institute study published in the current issue
of Nature Medicine.
The site where the egg lies is slightly warmer than the place
where the sperm make a pause in their journey through the
female genital tract. Sperm cells are apparently guided by this
temperature difference in their navigation. Such a
temperature-driven mechanism was previously known to exist in
micro-organisms and worms, but the Weizmann study, for the first
time, has provided evidence of its existence in sperm cells.
According to team leader Prof. Michael Eisenbach of the Institute's
Biological Chemistry Department, the study contributes to the
understanding of fertilization in humans and other mammals. In
the future, it may be possible to make use of temperature
guidance to improve in-vitro fertilization.
After passing the womb, sperm cells enter the fallopian tubes.
Once inside a tube, they attach themselves to the tube's wall and
pause for "storage," during which they go through a maturation
process that prepares them for penetrating the egg. A sperm that
has completed this maturation process detaches itself from the
wall and leaves the storage site. If ovulation had taken place in the
preceding 24 hours, releasing an egg ready to be fertilized, the
mature sperm would embark on a long, complicated journey
through the tube to the site of potential fertilization.
How does the sperm steer a course through the fallopian tube? In
the past, Prof. Eisenbach discovered that the egg "calls upon" the
mature sperm by releasing a chemical substance. (The first in a
series of Eisenbach's studies on that topic was published in the
Proceedings of the National Academy of Sciences, U.S.A.
1991;88:2840) However, the chemical signal can attract the sperm
only across a short range: since the tube normally moves in a
wave-like fashion, the chemical apparently cannot not spread
effectively through the entire tube and therefore cannot dispatch a
signal to the sperm over longer distances. Therefore, the chemical
attraction, known as "chemotaxis," cannot account for the sperm's
entire journey.
Some like it hot
To test this theory, the scientists built a laboratory installation
simulating the storage site, the fertilization site, and the tube in
between. They tested the behavior of rabbit sperm in this system
and found that the sperm were indeed sensitive to heat: they were
attracted from the relatively cool area, with the temperature of 37 C
(98.6 F), to the relatively warm area, with the temperature of 39 C
(102.2 F). When the scientists gradually reduced the difference in
temperature, they found that even a half a degree difference was
enough to attract the sperm. Moreover, the researchers found that
only the mature sperm, the ones likely to penetrate the egg, acted
as heat-guided missiles and were affected by the temperature
difference.
Says Prof. Eisenbach: "Apparently, the sperm are guided by
temperature when they travel through most of the fallopian tube
and navigate by tuning in to the egg's chemical call when they get
close to the fertilization site".
What then allows the sperm cells to cover the distance between
their storage site and the fertilization site within several minutes?
That's precisely the question that Prof. Eisenbach and his team
set out to answer. Since it was found that the sperm storage site is
about 2 C cooler than the site of fertilization, the scientists
theorized that sperm may be attracted to the fertilization site by the
temperature difference. The technical term for such attraction is
"thermotaxis."